1. Field of the Invention
This invention relates to a synthetic resin film having a uniform thickness, a method of removing one-sided material to obtain a uniform thickness upon production of such film and an apparatus for carrying out the method.
2. Description of the Prior Art
Upon production of a film wherein molten resin is drawn out from a narrow lip of a film molding die into a thin film, a film produced somtimes becomes non-uniform in thickness.
To remove such one-sided material upon molding of a film of the molten material extruded type, there are two methods including a method of varying the distance of a lip to mechanically control the thickness of a film and the other method of varying the temperature distribution of a lip to thermally control the thickness of a film.
The method of mechanically adjusting the distance of a lip to remove one-sided material has a limitation in accuracy and requires much time in adjustment to lead low response.
While it is well know that one-sided material can be removed by varying the temperature distribution of a lip, neither method nor apparatus for quickly adjusting the temperature distribution of a lip in response to a degree of one-sided material of a film to remove the one-sided material has been put into practical use so far.
Factors which cause one-sided material in a process of production of a synthetic resin film are complicated, but are roughly divided into two types incuding a type wherein a dimension or a configuration of a mechanical structure is a parameter and another type wherein a temperature, a drawing speed, a speed of cooling wind or the like is a parameter.
It is difficult or impossible to remove one-sided material which is caused by a factor wherein an element of time such as a speed or a speed of wind is included as a parameter or wherein time is not directly included as a parameter but a temperature which changes momentarily is included as a parameter by changing the dimension or configuration of a film molding apparatus.
Meanwhile, upon production of a film by drawing out molding, drawing out of a film cannot be stopped even during adjustment of a film thickness or one-sided material, and a film portion which is produced during such adjusting period makes an unacceptable article. Accordingly, a time required for adjustment of one-sided material or a responding time of an adjusted parameter to a completed article should be minimized desirably.
Particularly, unless initialization upon starting of operation of a film producing machine is completed quickly so that a film of desired specifications can be produced in a short time, the yield will be low and the price of film products will be increased.
On the other hand, when various kinds of films are to be produced in small quantities, specifications of a film may be changed without stopping operation of a film producing machine. It is desired to attain such adjustment to remove one-sided material quickly by a simple operation so that products of desired specifications may be obtained rapidly.
Thus, while a conventional apparatus wherein a large number of heaters are provided around a lip of a film molding die to remove one-sided material is effective in principle to remove one-sided material, such large number of heaters must be controlled in a required manner in order to reform the one-sided material. Accordingly, it is expected for such means to be put into practical use.